The present invention relates to offshore pipeline laying operations and, more specifically, to methods and apparatus for purging the interior of a submerged pipeline and/or scanning a pipeline interior for obstructions or defects.
In performing conventional pipeline laying operations, a pipeline is suspended between a floating lay barge and the water bottom and the barge is advanced while paying out the pipeline. As the pipeline descends through the body of water, proper pipeline profile control must be maintained to keep the radius of curvature of the pipeline below its elastic bending limit. In the absence of sufficient profile control, overstressing of the pipeline can occur, resulting in the formation of cracks, buckles, and even ruptures in the pipeline.
In order to maintain proper profile control, dynamic tensioner units, stinger structures (i.e., pivoted, buoyant ramps), and other well-known pipeline supporting systems are in wide use (see, for example, U.S. Pat. No. 3,390,532 to Lawrence, issued July 2, 1960). However, such equipment may not always prevent overstressing of a pipeline during laying operations, particularly when rough seas are encountered. As a consequence, the formation of buckles or ruptures during offshore pipelaying operations sometime occur.
When a rupture occurs in a pipeline during a laying operation, the operator on board the pipeline lay barge may detect the presence of the rupture by noticing the increase in pipeline weight caused by flooding of the pipeline interior through the rupture. The presence of a rupture or buckle may also be detected through the use of a buckle detecting apparatus as disclosed, for example, in U.S. Pat. No. 3,739,591 to Jones, issued June 19, 1973, assigned to the assignee of the present invention and the disclosure of which is herein incorporated by reference.
When such a rupture or buckle has been detected, it is necessary to remove the damaged submerged portion of the line and continue laying operations.
After the damaged portion of the line has been removed (possibly by use of divers) the undamaged previously-laid pipeline portion needs to be reconnected with pipeline means being supplied from the lay barge so that the laying operation may continue.
This reconnection may be effected by raising a portion of the undamaged previously-laid pipeline up to the lay barge where it may be connected with the pipeline being supplied by the pipeline feeding means of the lay barge.
One technique for effecting such a repair operation is disclosed in U.S. Pat. No. 3,751,932 to Matthews (issued Aug. 8, 1973). The Matthews technique entails the purging of water from the pipeline portion to be raised through the use of a pipeline pig. A pipeline pig constitutes a sphere, plug, or other device which is movable through the interior of a pipeline in slidable peripheral engagement with the pipeline interior.
The Matthews technique contemplates a calculation of the amount of displacing fluid acting on the purging pig so as to enable an operator to determine the length of the purged portion of the pipeline requisite to the raising operation, i.e., a minimal portion only of the laid line is purged.
The present invention constitutes an improvement in relation to the Matthews repair technique in that a previously-installed pig receiving receptacle is utilized so as to eliminate the uncertainties and calculations attendant upon the Matthews partial purging technique.
Moreover, the present invention enables multiple pig elements to be sequentially passed through a submerged pipeline portion to achieve optimum purging or to achieve a multiplicity of pigging functions (i.e., checking the interior of the pipeline to insure it is free from buckles, purging, etc.).
In addition, the present invention contemplates a possible utilization of "telltale" or "tattletale" indications of successful pig launching and the ultimate positioning of pipeline pigging elements.
Further, the present invention is applicable to pigging operations where inspection of the pipeline interior--as opposed to repairing only--is involved.
It will also be appreciated that aspects of the invention involving pig receiving may be practiced where the launching of a pig is effected either from a submerged location or from a floating vessel and regardless of where a damaged pipeline section is removed (i.e., at a submerged site or on board a vessel).
Such improvements in the art and other disadvantages are attained through the present invention, particularly in relation to the preferred embodiments which will now be described.